In recent years, along with the wave of informatization, the prevalence of broadband lines (broadband), which are capable of communicating large-capacity information at high speed, has increased. In addition, as transmission apparatuses that transmit information to this broadband line, a router apparatus, a WDM (Wavelength Division Multiplexing) apparatus, and the like have been used. A plurality of signal-processing boards, in which a computing device such as an LSI, a storage device such as a memory, and the like are combined, are provided in the transmitting apparatuses and function to mutually connect respective lines.
A circuit, in which the computing device, the storage device, and the like are connected through an electrical connection, is provided in the respective signal-processing boards. However, in recent years, accompanying an increase in the amount of information that is processed, it is required for each substrate to transmit information at a significantly high throughput. However, accompanying an increase in speed of information transmission, problems such as crosstalk or high-frequency noise, and deterioration of an electrical signal have occurred. Therefore, a bottleneck occurs in the electrical interconnection, and thus it is difficult to improve the throughput of the signal-processing substrate. In addition, the same problems occur in supercomputers, large-scale servers, or the like.
On the other hand, an optical communication technique that transmits data using an optical carrier wave has been developed, and an optical waveguide has been developed as means for guiding the optical carrier wave from one point to another point. This optical waveguide includes a linear core portion and a clad portion that is provided to cover the periphery of the core portion. The core portion is formed from a material that is substantially transparent with respect to light of the optical carrier wave. The clad portion is formed from a material with a refractive index lower than that of the core portion.
In the optical waveguide, light that is incident from one end of the core portion is conveyed to the other end thereof while being reflected at the boundary with the clad portion. A light-emitting element such as a semiconductor laser is disposed on an incidence side of the optical waveguide. A light-receiving element such as a photodiode is disposed on an output side. Light that is incident from the light-emitting element propagates through the optical waveguide and is received by the light-receiving element. The communication is carried out based on a flickering pattern or a strong and weak pattern of the light that is received.
In a case where the electrical interconnection in the signal-processing substrate is substituted with the optical waveguide, it is expected that the above-described problems related to the electrical connection may be solved, and thus an additional high throughput of the signal-processing substrate may be realized.
However, when the electrical interconnection is substituted with the optical waveguide, an optical waveguide module which includes a light-emitting element and a light-receiving element that are optically connected to each other by the optical waveguide is used in order for an electrical signal and an optical signal to be mutually converted.
For example, PTL 1 discloses an optical interface including a printed board, a light-emitting element that is mounted on the printed board, and an optical waveguide that is provided on a lower surface side of the printed board. In addition, the optical waveguide and the light-emitting element are optically connected to each other via a through-hole that is formed in the printed board as a through-hole that transmits an optical signal.
However, in regard to the above-described optical interface, there is a problem in that optical coupling loss is large in optical coupling between the light-emitting element and the optical waveguide. Specifically, when signal light that is emitted from a light-emitting unit of the light-emitting element passes through the through-hole and is incident on the optical waveguide, the signal light radially diverges, and thus the signal light is not entirely incident on the optical waveguide. Therefore, a part of the signal light does not contribute to the optical communication and an increase in optical coupling loss is caused.